it looks like we did not update the example to work nicely with tools that also use JSX. Have a look at the normal properties-panel-extension and how it uses htm to wrap the JSX.
Basically, what is happening here, your JSX compiler for Angular is trying to compile the Component from the properties panel extension, which is a Preact component, and fails. By wrapping it with htm, you do not need a JSX compiler for it to work.
Let me know if the properties-panel-extension works for you.